1. Field of the Invention
This invention relates to an optical disk drive. In particular, the present invention relates to an optical disk drive that compensates for eccentricities in an optical disk by controlling the position of a positioner. The positioner is driven by a motor, and an optical head is mounted thereon.
2. Description of the Related Art
An optical disk drive reads and/or writes information on tracks formed concentrically or spirally on an optical disk. The optical disk is mounted for rotation upon a turntable, which is rotated by a spindle motor, and the optical disk is rotated at a constant rotation speed about an axis of the spindle motor. A track being accessed for a read-operation and/or a write-operation is sought and tracked by positioning a small spot illuminated by a laser beam. The positioning of the small spot is carried out by both (1) a fine actuator which effects the movement of an objective lens mounted along the path of the laser beam; and (2) a coarse actuator which effects the displacement of the positioner, upon which is mounted an optical head (which includes the fine actuator, the objective lens, and the like).
It is well known that the data track often rotates eccentrically about the axis due to a miss-alignment of the center hole of the disk with a guide on the turntable for locking the disk. The miss-alignment may result from deficiencies in the machining precision of the hole and/or the guide, positional errors in assembling the turntable with the spindle motor, and/or errors in forming the track on the disk. As eccentric rotation of the track is often inherent in optical disk drives as shown above, it is necessary to compensate for this eccentric rotation when tracking the laser beam onto the desired track when data is being read and/or written from/to the disk.
There are two types of arrangements for compensating for the eccentric rotation. The first arrangement compensates for eccentric track-rotation by controlling the fine actuator which positions the small spot on the disk illuminated by the laser beam through the use of an objective lens that is moved by the fine actuator. The first arrangement is suited for use in disk drives for compact disks, which have larger track-pitches than those found in magneto-optical disks, and which also need to be inexpensive. The second arrangement compensates for the eccentric track-rotation by controlling both the fine actuator and the coarse actuator. The second arrangement is suited for use in disk drives for disks of high storage capacity, such as magneto-optical disks.
The compact disk drive having the first arrangement uses a DC motor (to lower the cost of the drive) as the coarse actuator. The DC motor is connected to the positioner by a gear train. When the eccentric rotation is compensated for by the fine actuator alone, the drive has the following problems. One problem is that of a residual position error in the fine actuator control, which results in a large error between the target position of the small spot and its actual position. A second problem is that large displacements of the objective lens for compensating for the eccentricity cause the optical axis of the objective lens to be inclined and shift, resulting in the laser beam illuminating an area off the desired track. These problems result in adversely affecting the C/N ratio (Carrier to Noise ratio), which indicates a characteristic of the write-and-read operations.
In high storage capacity disk drives, such as drives for disks having fine track pitches like magneto-optical disks, the eccentric displacement of the track is compensated for mainly by positioning the positioner. The movement of the objective lens is, hence, smaller than that found in compact disk drives. While such high capacity drives do not have the problems found in compact disk drives, these drives are expensive, in part because of the use of coarse actuators formed by large-size VCMs (Voice Coil Motors), which are made with expensive permanent magnets.
Because optical disk drives employ removable optical disks having higher data storage capacities than other removable recording media, the optical disk drive is indispensable as an external storage unit for use in information processing apparatuses. To increase the popularity of optical disks, it is necessary for the disk drive to be able to compensate for the above inherent eccentricity, and to be able to operate with disks of a high storage capacity without significantly increasing the cost of the drive.
An object of this invention is to provide an optical disk drive having an improved arrangement for compensating for eccentricities of the data tracks of the optical disk when the optical disk is rotated. In one preferred embodiment, the drive has a positioner for moving an objective lens, a motor for moving the positioner via a series of transmission gearing, a tracking error detector, an index pulse generating means, and an eccentric displacement data analyzing unit. The tracking error detector receives light reflected from the disk. The detector, hence, receives the light beam and detects a signal that includes information indicative of the number of track lines that have been crossed by the light beam when the objective lens is located in a same position. In the eccentric displacement data analyzing unit, a function indicative of the eccentricity of the disk rotation is analyzed from the signal and an index pulse supplied from the index pulse generating means, and this signal is stored in the eccentric displacement analyzing unit. A motor drive signal, which has been compensated for by the function, is supplied to the motor while the data track is being tracked by the light beam.
Other objects and advantages of the present invention will be apparent from the following description, the appending claims and accompanying drawings.